This invention relates to a thin film transistor (TFT) and a method of manufacturing the same, and more particularly to a TFT suitably used in an active matrix type liquid crystal display device and its manufacturing method.
The thin film transistor is generally used in electronic devices such as the liquid crystal display device (LCD) and a large area image sensor. Specifically, the TFT having source and drain electrodes, a gate electrode, and a channel region has been used as an element applying a voltage responsive to image information to a pixel electrode in an active matrix type liquid crystal display panel. In a transmissive active matrix LCD, a light shield film is provided for every TFT of such structure in order to prevent light from a backlight system or the like arranged on the side of a back face of the display panel from entering into the TFT, especially into a channel region thereof.
For instance, even when the TFT is controlled in a complete OFF condition, a degree of insulation between the source and the drain may be degraded to generate a leakage current if light impinges on the channel region. For this reason, an electric potential of the pixel electrode connected with the drain fluctuates unnecessarily, so that a deterioration of displayed-image quality is caused. In order to prevent such leakage current, the light shield film covering the channel region is formed on the side of the backlight system of the liquid crystal display panel, whereby a countermeasure that light can not impinge on the channel region has been taken.
In a method of manufacturing a TFT described in Japanese Laid-open Patent Application No. 131021/95, a silicon film doped with phosphorus (P), which is to be the light shield film is made on a glass substrate, and a silicon oxide film is made on the upper surface of the P-doped silicon film. Subsequently, an amorphous silicon film which is to be the source, the drain and the channel region in a subsequent process is made on an upper surface of the silicon oxide film. Furthermore, the silicon oxide film and an aluminum film are made on the upper surface side in order. Subsequently, an insular-laminated portion consisting of the aluminum film, the silicon oxide film and the amorphous silicon film is formed by a patterning process comprising a resist process (a masking process) and an etching process, the insular-laminated portion being narrower than the other films. Ions of nitrogen are then implanted into portions of the silicon oxide film and the phosphorus-doped silicon film in an area except for the laminated portion. Subsequently, an annealing process is performed only to the ion-implanted portions in the silicon oxide film and the silicon film doped with phosphorus to nitride them so as to make them transparent.
The well-known method of manufacturing the TFT provides an optically-transparentizing process for the desired portions as mentioned above, as well as it makes the phosphorus-doped silicon film on the under side of the laminated portion into a light shield film. The amorphous silicon film functioning as a channel region can not receive the light from an outer surface side of the glass substrate because the incidence of the light is shielded by the light shield film. Therefore, it is possible to prevent light from a backlight system from impinging on the finished channel region to generate a leakage current as mentioned above.
However, in such a prior art, there has been another aspect that the masking process using a dedicated photoresist mask for making the light shield film and subsequent exposing process and etching process or the like are required for the patterning process of exclusively forming the light shield film, whereby the burden on the manufacturing process of TFTs is heavy. For this reason, it has been disadvantageous from the viewpoint of especially the manufacturing cost.
In view of the circumstances mentioned above, the object of the invention is to provide a thin film transistor and a method of manufacturing the same which can reduce the burden on the manufacturing process and require an inexpensive manufacturing cost.
To this end, a thin film transistor in one aspect of the invention comprises a source electrode, a drain electrode and a gate electrode, the transistor further comprising: a light shield film formed on a substrate; and a semiconductor channel region formed on the light shield film, the region being shaped as same as the light shield film, the source electrode and the drain electrode being located on both sides of the light shield film and channel region in contact with the channel region.
According to this construction, it is possible to pattern and form the light shield film in the same shape as and simultaneously with the channel region on the common manufacturing stage. Therefore, commonality of a step of forming the light shield film and a step of forming the channel region can lead to simplification of the TFT manufacturing process and to an inexpensive manufacturing cost.
In the thin film transistor, the source electrode and the drain electrode may be formed in contact with both end-portions of an upper surface of the channel region, respectively.
The light shield film may consist of semiconductor or insulator. This choice of material can yield a light shield film suitable to the construction of the invention.
In addition, the source electrode and drain electrode are preferably in ohmic contact with the channel region. Such ohmic contact allows the source and drain to touch the channel region at a low contact-resistance.
A display device of the other aspect of the invention, uses a thin film transistor comprising a source electrode, a drain electrode and a gate electrode, the thin film transistor comprising: a light shield film formed on a substrate; and a channel region of semiconductor formed on the light shield film, the region being shaped as same as the light shield film.
A method of manufacturing a thin film transistor, according to the other aspect of the invention, comprising: a first deposition step of depositing a light shield film on a substrate; a second deposition step of depositing a semiconductor film for forming a channel of the transistor on the light shield film; a patterning step of shaping the light shield film and the semiconductor film simultaneously into the same shape pattern; an electrode forming step of forming a source electrode and a drain electrode which are respectively in contact with both end-portions of the shaped semiconductor film; a step of forming an insulator film so that the insulator film covers the source and drain electrodes, and the semiconductor film; and a step of forming a gate electrode at a location on the insulator film, corresponding to the semiconductor film.
In the method, an ohmic contact treatment step may be performed for doping a surface layer portion of the semiconductor film with phosphorus after the second deposition step and before the patterning step.
The patterning step may include a process for masking stacked layers of the light shield film and the semiconductor film by using a single mask.
After the electrode forming step an insulation treatment step may be performed for nitriding or oxidizing a surface layer portion of the semiconductor film which is not covered with the source electrode and the drain electrode.